Band reject filters

ABSTRACT

A method and a band reject filter (BRF) using as acoustic resonators at least one of bulk acoustic wave (BAW) resonators and film bulk acoustic resonators (FBAR) are provided. The BRF includes at least one substrate having at least one of a plurality of capacitors formed thereon, the plurality of capacitors having capacitances selected to achieve a particular band reject response. The BRF also includes at least one die. At least one of a plurality of acoustic wave resonators are formed thereon. The plurality of acoustic wave resonators are one of BAW resonators and FBARs and are designed to have the same resonant frequency. A plurality of conductors between the substrate and the die are positioned to electrically connect the acoustic wave resonators and the capacitors.

TECHNICAL FIELD

This disclosure relates to band reject filters.

BACKGROUND

Band reject filters (BRFs) are used in devices such as wireless networksbase stations and wireless devices to combat interference encountered onco-location/co-site radio deployments, as well as on multi-band radiodesigns. In particular, demand for BRFs that are small in size, light inweight and low in cost is very strong, because of interference thatoccurs in cell networks such as fourth generation (4G) small cellnetworks.

Typically, BRFs used to mitigate interference require a steep transitionband between the passband and the reject band. To achieve BRFs with suchsteep transition bands, a high Q resonator is desired. Surface acousticwave (SAW) resonators have a relatively high Q of about 1000 around a 1giga-Hertz (GHz) resonance frequency. The Q of a SAW resonator drops toabout 500 when its resonance frequency is up to 2 GHz. Since a Q of 500is not high enough to meet BRF filtering requirements in wirelessnetworks applications, the SAW resonator-based BRF is useful to counterinterference within a frequency range of 0.5 to 1.5 GHz.

Bulk acoustic wave (BAW) resonators and film bulk acoustic resonators(FBAR) are high Q devices that may be used in BRFs. They have muchhigher Q than SAW resonators. For example, a BAW resonator or FBAR mayhave a Q as high as 3000 at 2 GHz, and even show a Q of about 2000 at afrequency of 3 GHz. Thus, from the standpoint of high Q, both the BAWresonator and the FBAR are desirable.

However, the BAW resonator and the FBAR have very complicatedmanufacturing processes in comparison to a SAW resonator and are up to50-100 times more expensive to design than SAW resonators. Such largeexpense is a chief reason why BAW resonators and FBARs are not widelyused in wireless networks base stations designs because of their smallamount of needs.

SUMMARY

Some embodiments advantageously provide band reject filters and thedesign thereof. According to one aspect, a band reject filter (BRF)using as acoustic resonators at least one of bulk acoustic wave (BAW)resonators and film bulk acoustic resonators (FBAR) is provided. The BRFincludes at least one substrate having at least one of a plurality ofcapacitors formed thereon, the plurality of capacitors havingcapacitances selected to achieve a particular band reject response. TheBRF also includes at least one die. At least one of a plurality ofacoustic wave resonators are formed on the at least one die. Each of theplurality of acoustic wave resonators are one of BAW resonators andFBARs and designed to have the same resonant frequency. A plurality ofconductors between the at least one substrate and the at least one dieare positioned to electrically connect the acoustic wave resonators andthe capacitors.

According to this aspect, in some embodiments, the at least onesubstrate, the at least one die, acoustic wave resonators, capacitorsand conductors are enclosed in a sealed package. In some embodiments,the conductors are solder balls or gold bumps. In some embodiments,there are n resonators electrically in parallel and there are mresonators electrically in series, n and m being integers. In someembodiments, each of the n resonators are electrically in series with acapacitor. In some embodiments, each of the m resonators areelectrically in parallel with a capacitor. In some embodiments, the BRFincludes a plurality of inductors electrically in parallel with the nresonators. In some embodiments, each of the plurality of capacitors isan interdigital-type capacitor with a number of fingers selected toadjust a capacitance of the capacitor. In some embodiments, thecapacitances are chosen to compensate for a deviation of an acousticwave resonator from the designed resonant frequency.

According to another aspect, a band reject filter IC is provided. TheBRF includes at least one die. The BRF also includes a plurality ofacoustic wave resonators, the plurality of acoustic wave resonatorsbeing one of BAW, resonators and FBARs. The plurality of acoustic waveresonators are formed on the at least one die and are designed to havethe same resonant frequency. The BRF also includes at least onesubstrate and formed on the at least one substrate, a plurality ofcapacitors, the plurality of capacitors having capacitances selected toachieve a particular band reject response. Electrical conductorspositioned between the at least one die and the at least one substrateelectrically connect the capacitors on the at least one substrate to atleast one acoustic wave resonators on the at least one die.

According to this aspect, in some embodiments, the at least onesubstrate, the at least one die, the acoustic wave resonators and theconductors are enclosed in a sealed package. In some embodiments, theconductors are solder balls or gold bumps. In some embodiments, are nresonators electrically in parallel and there are m resonatorselectrically in series, n and m being integers. In some embodiments,each of the n resonators are electrically in series with a capacitor. Insome embodiments, each of the m resonators are electrically in parallelwith a capacitor. In some embodiments, the BRF IC includes a pluralityof inductors electrically in parallel with the n resonators. In someembodiments, each of the plurality of capacitors is an interdigital-typecapacitor with a number of fingers selected to adjust a capacitance ofthe capacitor. In some embodiments, the capacitances are chosen tocompensate for a deviation of an acoustic wave resonator from thedesigned resonant frequency

According to yet another aspect, a method of manufacturing a band rejectfilter, BRF, is provided. The method includes forming a plurality ofacoustic wave resonators on a die, each of the plurality of acousticresonators being designed to have the same resonant frequency, eachacoustic wave resonator being one of bulk acoustic wave, BAW, resonatorsand film bulk acoustic resonators, FBAR. Capacitors are formed on asubstrate. The capacitors have capacitances chosen to achieve aparticular band reject response. A flip-chip manufacturing technique isemployed to mate the die and the substrate with solder balls or goldbumps to electrically connect the capacitors on the substrate to atleast one corresponding acoustic resonator on the die.

According to this aspect, the method also includes sealing the die, thesubstrate, the acoustic wave resonators and the capacitors in anintegrated circuit package. In some embodiments, each of the pluralityof capacitors is an interdigital-type capacitor with a number of fingersselected to adjust a capacitance of the capacitor. In some embodiments,the method further includes packaging the at least one substrate, the atleast one die, acoustic wave resonators, capacitors and conductors in asealed integrated circuit package. In some embodiments, the capacitancesare chosen to compensate for a deviation of an acoustic wave resonatorfrom the designed resonant frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present embodiments, and theattendant advantages and features thereof, will be more readilyunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic diagram of a known band reject filter (BRF);

FIG. 2 is a graph of band reject responses for the BRFs shown in FIGS. 1and 3;

FIG. 3 is a schematic diagram of a known BRF with tuning capacitors;

FIG. 4 is a schematic of a known BRF having six differently designedacoustic resonators;

FIG. 5 is an illustration of a known package including the sixdifferently designed acoustic resonators;

FIG. 6 is a schematic diagram of a BRF according to methods presentedherein;

FIG. 7 is an illustration of a BRF package according to methodsdescribed herein;

FIG. 8 is an illustration of an assembly of a BRF using a flip-chiptechnology according to methods described herein;

FIG. 9 is an illustration of interdigital capacitors on a substrate thatmay be integrated with a BRF according to method described herein;

FIG. 10 is a schematic diagram of a BRF according to method presentedherein;

FIG. 11 is a graph of two band reject responses resulting from twodifferent BRF designs using the same resonators; and

FIG. 12 is a flowchart of an exemplary process for design of a BRFaccording to methods described herein.

DETAILED DESCRIPTION

Before describing in detail exemplary embodiments, it is noted that theembodiments reside primarily in combinations of apparatus components andprocessing steps related to band reject filter implementation.Accordingly, components have been represented where appropriate byconventional symbols in the drawings, showing only those specificdetails that are pertinent to understanding the embodiments so as not toobscure the disclosure with details that will be readily apparent tothose of ordinary skill in the art having the benefit of the descriptionherein.

As used herein, relational terms, such as “first” and “second,” “top”and “bottom,” and the like, may be used solely to distinguish one entityor element from another entity or element without necessarily requiringor implying any physical or logical relationship or order between suchentities or elements.

FIG. 1 is a schematic diagram of a typical BRF design using acousticresonators. Each of the three different resonators of FIG. 1 is designedto have a different resonance frequency to form the width of the bandreject region, as shown in FIG. 2 curve 10. For a BAW resonator or anFBAR, a different resonance frequency means a different design for eachresonator. A different design requires a different combination ofthicknesses of an active piezoelectric layer and mass loading metallayer. To change the thickness of these layers, a certain number ofprocessing steps in sequence are needed for the manufacture of theresonator, which results in increased cost.

Currently, only one piezoelectric material is used for the design of BAWresonators and FBARs, which is aluminum nitride (AIN). Such resonatorshave an electromechanical coupling coefficient of about 6 or 6.5%. Forsome BRF designs, such values are too high and must be reduced by use ofcapacitors, so that a sharp transition band can be designed. This isshown in FIG. 3 where capacitors, Cp1 and Cp2, are placed in parallelwith the two series resonators, Res1 and Res3, and a capacitor, Cs1, isplaced in series with the parallel resonator, Res2.

As shown in FIG. 2, curve 10, the stopband region of the BRF band rejectresponse of the circuit of FIG. 1 has three valleys, one for each of thethree different resonators of the BRF of FIG. 1. A sharper transitionregion can be achieved using the capacitors shown in the circuit of FIG.3. This sharper transition region is shown as curve 12 of FIG. 2. Notethat in FIG. 2, the area 11 between the shaded regions is a transitionregion of the band reject response.

A more general BRF design is shown in FIG. 4. The design shown in FIG. 4has six different resonators and six capacitors. As shown in FIG. 5,conventionally, the six resonators are included in a package and thecapacitors and matching inductors are added as peripheral componentsoutside the package. The placement of the capacitors peripheral to thepackage results in the consumption of large spaces of the printedcircuit board on which the package is placed.

In summary, conventional designs using BAW resonators and FBARs areexpensive, involving the design of many different resonators, and use oflarge board space for peripheral components. Also, since the placementof the different resonant frequencies of the resonators may be critical,repeated manufacture may be required to achieve a critical resonantfrequency and/or transition region, thereby increasing the cost ofmanufacture even further.

To address the above, embodiments of the present disclosure provide BRFsusing BAW resonators and FBARs, and design methodologies for the designof BRFs using BAW resonators and FBARs, which reduce design costs,manufacturing costs and size as compared with known BRFs. FIG. 6 is aschematic of a design in which all the acoustic resonators are designedto have the same resonant frequency. Rather than design a differentresonator for each resonator section of the BRF circuit to achievedifferent valleys within the band reject region, each of the resonators,Res1, of the BRF are designed to resonate at the same frequency, and thewidth of the band reject region is achieved by selection of thecapacitors Cp1-Cpm and Cs1-Csn, where there are m series resonators andn parallel resonators. Each of the m series resonators has a capacitorCp electrically in parallel with the corresponding resonator and each ofthe n parallel resonators has a capacitor Cs electrically in series withthe corresponding resonator. Capacitors Cp and Cs are referred to hereincollectively as capacitors 30.

By using the same design for each resonator in the BRF, substantialdesign and manufacturing costs may be avoided. The capacitances of thecapacitors may be chosen to compensate for a deviation of an acousticwave resonator from the designed resonant frequency. In someembodiments, each capacitor may be an interdigital-type capacitor with anumber of fingers selected to adjust a capacitance of the capacitor. Insome embodiments, the capacitors are formed on a high Q substrate.

FIG. 7 is an illustration of a BRF in a BRF package 18 that includes notonly the n+m resonators having the same design, but also include the n+mcapacitors as well. Including the capacitors in the BRF package 18 withthe resonators substantially reduces the amount of board space utilizedfor peripheral components. Note that the matching inductors 19 remainoutside the BRF package 18 in some embodiments. In one embodiment, theBRF package is an integrated circuit (IC), also referred to herein as aBRF IC.

FIG. 8 is an illustration of an example showing how the resonators andcapacitors may be integrated in the same BRF package 18 using flip chiptechnology. All of the resonators 21 in the BRF package 18 may be formedon a single die 20, and all of the capacitors 23 may be formed on asingle substrate 22. Connections between the capacitors 23 andresonators 21 may be formed using metal bumps 24, which may be solderballs or gold bumps, for example Note that an exterior surface of thedie 20 or the substrate 22 may form a surface of the BRF package 18 orthe entire die 20 and/or substrate 22 may be fully enclosed within theBRF package 18.

Thus, some embodiments include a BRF using as acoustic resonators atleast one of BAW resonators and FBARs. The BRF includes a substrate anda die. A plurality of acoustic wave resonators are formed on the die.The plurality of acoustic resonators are of the same designed resonantfrequency. A plurality of capacitors are formed on the substrate. Theplurality of capacitors have capacitances selected to achieve aparticular band reject response. Electrical conductors, such as metalbumps or solder balls or gold bumps lie between the die and thesubstrate and are positioned to electrically connect the acoustic waveresonators and the capacitors. In one embodiment, the components of theBRF may be a sealed package such as in an integrated circuit (IC)package. In some embodiments, the acoustic wave resonators may be formedon more than one die, and the capacitors may be formed on more than onesubstrate. The acoustic wave resonators, the capacitors, thesubstrate(s) and the die(s) may be packaged in a sealed integratedcircuit package.

FIG. 9 is an illustration of capacitors 30 formed on a dielectricsubstrate 22 such as an alumina substrate, Al₂O₃, or single crystal ormulti-crystal substrates such as Quartz, LiNbO3, LiTaO3, sapphire,silicon, as well as some printed circuit board (PCB) materials such asFR-4 and Rogers, etc. In one embodiment, the dielectric materials usedmay have small loss tangent to allow the capacitors to have Q valueshigh enough to achieve a desired band reject transition region. Notethat the capacitors may be aligned symmetrically or placed in anasymmetric pattern, as desired.

Electrode patterns of the capacitors 30 (Cp1 . . . Cpm and Cs1 . . .Csn) on the substrate 22 may be any shape but an interdigital pattern asshown in FIG. 9 may be used. This type of capacitor can be manufacturedto within 0.01 pico-Farad (pF) of capacitance. The interlaced fingers ofthe interdigital capacitor 30 may have any desired pitch (spacing)except that when a SAW substrate, such as, quartz or LiTaO3 is used, thepitch of the interdigital capacitor should be a value that issubstantially different from a value of p=v/(2fo), where v is velocityof propagation of the SAW (surface acoustic wave) along a directionperpendicular to the fingers of the interdigital capacitor and fo is thecenter frequency of the reject band of the BRF. This condition avoidsgeneration of a surface acoustic wave on the substrate when signalswithin the reject band are passing through the capacitor.

FIG. 10 is a schematic diagram of a BRF with three resonators 32 a, 32 band 32 c (collectively “resonators 32”), all designed to have the sameresonant frequency. Electrically parallel with the series resonators 32b and 32 c are capacitors 30 b and 30 c. In electrical series with theelectrically parallel resonator 32 a is a capacitor 30 a. Electricallyin parallel with the electrically parallel resonator 32 a is a matchinginductor 40. Of course, implementations are not limited to thearrangement shown in FIG. 10. Other quantities of resonators andcapacitors may be used. FIG. 11 is a graph of two band reject responses42 and 44. Response 42 corresponds to the circuit of FIG. 10 when thecomponents have the following values:

Cp1=1.28 pF, Cp2=1.60 pF, Cs1=0.70 pF, L1=4.3 nH

Response 44 corresponds to the circuit of FIG. 10 when the componentshave the following values:

Cp1=0.73 pF, Cp2=1.04 pF, Cs1=0.46 pF, L1=4.3 nH

Note that in FIG. 11, the area between the shaded regions is atransition region 45 of the band reject responses 42 and 44. Note alsothat the values of the capacitances and inductor are not difficult toachieve. In particular, the capacitors may be formed usingphotolithography.

FIGS. 10 and 11 show that the same resonators Res1 with differentcapacitances can achieve quite different band reject responses. In otherwords, all the resonators can be designed to have the same resonantfrequencies and the valleys in the band reject response can be adjustedby varying the capacitors of the circuit. Thus, not only is the cost ofdesign reduced because the same design can be used for all theresonators, but the manufacturing tolerances on the resonator design maynot be critical since the response can be fine-tuned by varying thecapacitors. Implementing capacitors of varied capacitances can be doneat a low cost.

Therefore, embodiments include a method of manufacturing a band rejectfilter using BAW resonators or FBARs that is simple and low in cost.First, a plurality of acoustic wave resonators are formed on a die. Eachof the resonators are of a same design, namely designed to have a sameresonant frequency. Capacitors are chosen having capacitances to achievea particular band reject response. Capacitors having the chosencapacitances are formed on a substrate. Flip chip technology can be usedto mate the die and the substrate with solder balls or gold bumpsforming connections between the capacitors and the acoustic resonators.The assembled die and substrate can be enclosed in a sealed integratedcircuit (IC) package.

FIG. 12 is a flowchart of an exemplary process for manufacturing a bandreject filter, BRF, using acoustic wave resonators chosen from a groupconsisting of bulk acoustic wave, BAW, resonators and film bulk acousticresonators, FBAR. The method includes forming a plurality of acousticwave resonators on a die, each of the plurality of acoustic resonatorshaving the same resonant frequency (block S100). The capacitors areformed on a substrate and have a predetermined capacitance chosen toachieve a particulate band reject response (block S102). A flip-chipmanufacturing technique is employed to mate the die and the substratewith solder balls or gold bumps to electrically connect the capacitorson the substrate to at least one acoustic resonator on the die (blockS108).

The embodiments described herein enable a filter designer to design arelatively low-cost BRF using BAW resonators or FBARs with a shortdesign time. The BRFs described herein can be used in embodiments suchas base stations and hand held wireless devices. Since BRFs using BAWresonators and FBARs have much better filtering performance than SAWresonators, BRFs using BAW resonators and FBARs can be expected to helpnetwork operators use their valuable spectrum resources moreefficiently.

As will be appreciated by one of skill in the art, the conceptsdescribed herein may be embodied as a method, data processing system,and/or computer program product. Accordingly, the concepts describedherein may take the form of an entirely hardware embodiment, an entirelysoftware embodiment or an embodiment combining software and hardwareaspects all generally referred to herein as a “circuit” or “module.”Furthermore, the disclosure may take the form of a computer programproduct on a tangible computer usable storage medium having computerprogram code embodied in the medium that can be executed by a computer.Any suitable tangible computer readable medium may be utilized includinghard disks, CD-ROMs, electronic storage devices, optical storagedevices, or magnetic storage devices.

Some embodiments are described herein with reference to flowchartillustrations and/or block diagrams of methods, systems and computerprogram products. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computerreadable memory or storage medium that can direct a computer or otherprogrammable data processing apparatus to function in a particularmanner, such that the instructions stored in the computer readablememory produce an article of manufacture including instruction meanswhich implement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer orother programmable data processing apparatus to cause a series ofoperational steps to be performed on the computer or other programmableapparatus to produce a computer implemented process such that theinstructions which execute on the computer or other programmableapparatus provide steps for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

It is to be understood that the functions/acts noted in the blocks mayoccur out of the order noted in the operational illustrations. Forexample, two blocks shown in succession may in fact be executedsubstantially concurrently or the blocks may sometimes be executed inthe reverse order, depending upon the functionality/acts involved.Although some of the diagrams include arrows on communication paths toshow a primary direction of communication, it is to be understood thatcommunication may occur in the opposite direction to the depictedarrows.

Computer program code for carrying out operations of the conceptsdescribed herein may be written in an object oriented programminglanguage such as Java® or C++. However, the computer program code forcarrying out operations of the disclosure may also be written inconventional procedural programming languages, such as the “C”programming language. The program code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer. In the latter scenario, theremote computer may be connected to the user's computer through a localarea network (LAN) or a wide area network (WAN), or the connection maybe made to an external computer (for example, through the Internet usingan Internet Service Provider).

Many different embodiments have been disclosed herein, in connectionwith the above description and the drawings. It will be understood thatit would be unduly repetitious and obfuscating to literally describe andillustrate every combination and subcombination of these embodiments.Accordingly, all embodiments can be combined in any way and/orcombination, and the present specification, including the drawings,shall be construed to constitute a complete written description of allcombinations and subcombinations of the embodiments described herein,and of the manner and process of making and using them, and shallsupport claims to any such combination or subcombination.

It will be appreciated by persons skilled in the art that theembodiments described herein are not limited to what has beenparticularly shown and described herein above. In addition, unlessmention was made above to the contrary, it should be noted that all ofthe accompanying drawings are not to scale. A variety of modificationsand variations are possible in light of the above teachings withoutdeparting from the scope of the following claims.

What is claimed is:
 1. A band reject filter, BRF, the BRF comprising: atleast one substrate, the at least one substrate having at least one of aplurality of capacitors formed thereon, the plurality of capacitorshaving capacitances selected to achieve a particular band rejectresponse; at least one die, the at least one die having at least one ofa plurality of acoustic wave resonators formed thereon, the plurality ofacoustic wave resonators being one of bulk acoustic wave, BAW,resonators and film bulk acoustic resonators, FBAR, and each of theplurality of acoustic wave resonators being designed to have a sameresonant frequency; and a plurality of conductors between the at leastone substrate and the at least one die and positioned to electricallyconnect the acoustic wave resonators and the capacitors.
 2. The BRF ofclaim 1, wherein the at least one substrate, the at least one die,acoustic wave resonators, capacitors and conductors are enclosed in asealed package.
 3. The BRF of claim 1, wherein the conductors are solderballs or gold bumps.
 4. The BRF of claim 1, wherein there are nresonators electrically in parallel and there are m resonatorselectrically in series, n and m being integers.
 5. The BRF of claim 4,wherein each of the n resonators are electrically in series with acapacitor.
 6. The BRF of claim 4, wherein each of the m resonators areelectrically in parallel with a capacitor.
 7. The BRF of claim 4,further comprising a plurality of inductors electrically in parallelwith the n resonators.
 8. The BRF of claim 1, wherein each of theplurality of capacitors is an interdigital-type capacitor with a numberof fingers selected to adjust a capacitance of the capacitor.
 9. The BRFof claim 1, wherein the capacitances are chosen to compensate for adeviation of an acoustic wave resonator from the designed resonantfrequency.
 10. A band reject filter, BRF, integrated circuit, IC,comprising: at least one die; a plurality of acoustic wave resonators,the plurality of acoustic wave resonators being one of bulk acousticwave, BAW, resonators and film bulk acoustic resonators, FBAR, theplurality of acoustic wave resonators being formed on the at least onedie and being designed to have a same resonant frequency; at least onesubstrate; a plurality of capacitors, the plurality of capacitors beingformed on the at least one substrate and having capacitances selected toachieve a particular band reject response; and electrical conductorspositioned between the at least one die and the at least one substrate,the electrical conductors arranged to electrically connect thecapacitors on the at least one substrate to at least one correspondingacoustic wave resonator on the at least one die.
 11. The BRF IC of claim10, wherein the at least one substrate, the at least one die, theacoustic wave resonators and the conductors are enclosed in a sealedpackage.
 12. The BRF IC of claim 10, wherein the electrical conductorsare solder balls or gold bumps.
 13. The BRF IC of claim 10, whereinthere are n resonators electrically in parallel and there are mresonators electrically in series, n and m being integers.
 14. The BRFIC of claim 13, wherein each of the n resonators are electrically inseries with a capacitor.
 15. The BRF IC of claim 13, wherein each of them resonators are electrically in parallel with a capacitor.
 16. The BRFIC of claim 13, further comprising a plurality of inductors electricallyin parallel with the n resonators.
 17. The BRF IC of claim 10, whereineach of the plurality of capacitors is an interdigital-type capacitorwith a number of fingers selected to adjust a capacitance of thecapacitor.
 18. The BRF IC of claim 10, wherein the capacitances arechosen to compensate for a deviation of an acoustic wave resonator fromthe designed resonant frequency.
 19. A method of manufacturing a bandreject filter, BRF, the method comprising: forming a plurality ofacoustic wave resonators on a die, each of the plurality of acousticresonators being one of bulk acoustic wave, BAW, resonators and filmbulk acoustic resonators, FBAR, each of the acoustic wave resonatorsbeing designed to have a same resonant frequency; forming a plurality ofcapacitors having predetermined corresponding capacitances on asubstrate, the capacitances of capacitors being chosen to achieve aparticular band reject response; and employing a flip-chip manufacturingtechnique to electrically mate the die and the substrate using solderballs or gold bumps to electrically connect the capacitors on thesubstrate to at least one corresponding acoustic wave resonator on thedie.
 20. The method of claim 19, further comprising sealing the die, thesubstrate, the acoustic wave resonators and the capacitors in anintegrated circuit package.
 21. The method of claim 19, wherein each ofthe plurality of capacitors is an interdigital-type capacitor with anumber of fingers selected to adjust a capacitance of the capacitor. 22.The method of claim 19, further packaging the at least one substrate,the at least one die, acoustic wave resonators, capacitors andconductors in a sealed integrated circuit package.
 23. The method ofclaim 19, wherein the capacitances are chosen to compensate for adeviation of an acoustic wave resonator from the designed resonantfrequency.